Image blur is a common problem in both digital and traditional photography. While there are a variety of causes of image blur, one of the most common causes is camera shake inadvertently introduced by the operator of the camera. Image blur resulting from camera shake becomes more pronounced as cameras or other imaging devices become smaller and lighter, with longer exposure times, or when the operator uses a zoom or telephoto lens with a long focal length. Hand-held imaging devices, such as cameras or associated lenses, may include an image stabilization system to reduce image blur.
Generally, image stabilization systems detect motion of the imaging device and attempt to reduce blur by counteracting or compensating for the detected motion. Motion of the imaging device may be detected by the inclusion of a pair of micro-electro-mechanical (MEM) gyroscopes within the imaging device. Typically each MEM gyroscope detects motion in a plane. In digital photography, motion of the imaging device may be electronically detected by determining any movement of otherwise “still” objects within the field of view of successive frames captured by the imaging device.
In digital photography, once the motion is determined, compensation can be performed, for example, by moving a flexible platform to which a pixel array is mounted to compensate for the detected motion. Alternatively, elements of a lens structure may be moved relative to the array to compensate for the detected motion.
FIG. 1 is an illustration of a conventional four transistor (4T) pixel 100 typically used in a CMOS imager for example, in a digital camera. Pixel 100 functions by receiving photons of light reflected from a desired image and converting those photons into electrons. Pixel 100 includes a photosensor 105, shown as a photodiode, a floating diffusion charge storage region (floating diffusion region) 110, and four transistors: a transfer transistor 115, a reset transistor 120, a source follower transistor 125, and a row select transistor 130. The pixel 100 accepts a TX control signal for controlling the conductivity of the transfer transistor 115, a RST control signal for controlling the conductivity of the reset transistor 120, and a ROW control signal for controlling the conductivity of the row select transistor 130. The charge stored at the floating diffusion region 110 controls the conductivity of the source follower transistor 125. The output of the source follow transistor 125 is presented at node 135, which is connected to a column line of a pixel array, when the row select transistor 130 is conducting.
The pixel 100 is operated as follows. The ROW control signal is asserted to cause the row select transistor 130 to conduct. At the same time, the RST control signal is asserted while the TX control signal is not asserted. This connects the floating diffusion region 110 to the pixel power potential VAAPIX at node 140, and resets the voltage at this floating diffusion region 110 to the pixel power potential VAAPIX, less a voltage drop associated with reset transistor 120. The pixel 100 outputs a reset signal (Vrst) at node 135. As will be explained in greater detail below in connection with FIG. 2, node 135 is typically coupled to a column line 235 (FIG. 2) of an imager 200, which supplies a constant current through the source follower transistor 125.
While the transfer transistor 115 is off, the photosensor 105 is exposed to incident light focused by a lens or a lens system and accumulates charge based on the level of the incident light during what is often referred to as a charge integration period. After the charge integration period and after the RST control signal turns off reset transistor 120, the TX control signal is asserted. This transfers charge from the photosensor's charge accumulation region to the floating diffusion region 110 by connecting the floating diffusion region 110 to the photosensor 105. Charge flows through the transfer transistor 115 and lowers the voltage at the floating diffusion region 110 proportional to the accumulated charge and the capacitance of the floating diffusion node. The pixel 100 thus outputs a photo signal (Vsig) at node 135 which is connected to a column line of a pixel array.
The states of the transfer and reset transistors 115, 120 determine whether the floating diffusion region 110 is connected to the light sensitive element 105 for receiving photo-generated charge accumulated by the light sensitive element 105 after the charge integration period, or a source of pixel power VAAPIX from node 140 during the reset period. In use, the floating diffusion region 110 is typically blocked from sensing light or, if floating diffusion region 110 is not blocked from sensing light, any light generated charge collected by the floating diffusion region 110 is lost through the reset process.
FIG. 2 is an illustration of an imager 200 that includes a plurality of pixels 100 forming a pixel array 205. Typically, each pixel has a micro lens and a color filter in the light path to the pixel and the color pixels are arranged in a known Bayer R,G,B, pattern. Due to space limitations, the pixel array 205 is drawn as a 4 row by 4 column array in FIG. 2. One skilled in the art would recognize that most imagers 200 would ordinarily include hundreds, thousands, or millions of pixels 100 in the pixel array. The imager 200 also includes row circuitry 210, column circuitry 215, a digital processing circuit 220, and a storage device 225. The imager 200 also includes a controller 230, for controlling operations of the imager 200.
The row circuitry 210 operates a row of pixels 100 from the pixel array 205. The pixels 100 in the selected row output their reset and photo signals Vrst, Vsig to the column circuitry 215, via column output lines 235, which samples and holds the reset and photo signals Vrst, Vsig for each pixel in a row. The rows are activated one by one in sequence to send successive reset and photo signals from pixels of a selected row to the column output lines 235.
In prior art devices, the column circuitry 215 is responsible for converting the pixel reset Vrst and photo Vsig signals into digital values that can then be further processed in the digital domain. In order to do this, the column circuitry 215 samples and holds the reset Vrst and photo Vsig signals produced by each pixel. An analog pixel output signal (Vpixel) is formed as the difference between the reset Vrst and photo Vsig signals, e.g., Vpixel=Vrst−Vsig. Alternatively, the analog pixel output signal may be received from an analog readout to side analog to digital conversion. The pixel output signal Vpixel is then converted into a digital value representing the luminance of a pixel. Imager 200 uses a column parallel architecture, in which the outputs of several pixels 100 in the selected row are simultaneously sampled and held, and converted to digital values. The digital values are sent to the digital processing circuit 220, which performs image processing on the digital values to produce a digital image. The processed digital values are stored in the storage device 225. The controller 230 is coupled to the pixel array 205, row circuitry 210, column circuitry 215, and storage device 225, and provides control signals to perform the above described processing. The imager 200 is typically employed in a digital camera where image stabilization is a desirable feature. Past efforts to provide image stabilization in such a system have not been entirely satisfactory.